Generator protection

GRIDTechnical Institute

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Generator Protection

Generator Protection - P 2

The extent and types of protection specified will depend on the following factors :-

• Type of prime mover and generator construction

• MW and voltage ratings

• Mode of operation

• Method of connection to the power system

• Method of earthing

• Types of Prime Mover − Steam Turbines− Gas Turbines− Hydro− Diesel

• Construction− Cylindrial Rotor− Salient Pole (Hydro and small generators)

• Mode of operation− Base load− Peak lopping− Standby

• Ratings− Power from 200kVA to 1000MVA− Voltage from 440V to 24kV

Connection to the Power System

1. Direct :

2. Via Transformer :

Generator Protection Requirements

• To detect faults on the generator

• To protection generator from the effects of abnormal power system operating conditions

• To isolate generator from system faults not cleared remotely

• Action required depends upon the nature of the fault.

• Usual to segregate protection functions into :

− Urgent− Non-urgent− Alarm

Generator Faults

Mixture of mechanical and electrical problems.

Faults include :-

• Insulation Failure− Stator− Rotor

• Excitation system failure• Prime mover / governor failure• Bearing Failure• Excessive vibration• Low steam pressure• etc.

System Conditions

• Short circuits• Overloads• Loss of load• Unbalanced load• Loss of synchronism

Generator Failure

Stator Earth Fault Protection

Fault caused by failure of stator winding insulation

Leads to burning of machine corewelding of laminations

Rebuilding of machine core can be a very expensive process

Earth fault protection is therefore a principal feature of any generator protection package

TYPE OF METHOD METHODPROTECTION OF OF

EARTHING CONNECTION

Method of Earthing

Machine stator windings are surrounded by a mass of earthed metal

Most probable result of stator winding insulation failure is a phase-earth fault

Desirable to earth neutral point of generator to prevent dangerous transient overvoltages during arcing earth faults

Several methods of earthing are in use Damage resulting from a stator earth fault will

depend upon the earthing arrangement

Method of Earthing

Solidly Earthed Machines :

• Fault current is high

• Rapid damage occurs− burning of core iron− welding of

laminations

• Used on LV machines only

Generator - Transformer Units

IF ~ 200 300 A

IF ~ 10 15 A

Method of Earthing

Desirable to limit earth fault current :

− limits damage− reduces possibility of developing into phase -

phase fault

Degree to which fault current is limited must take into account :

− detection of earth faults as near as possible to the neutral point

− ease of discrimination with system earth fault protection (directly connected machines)

Method of Earthing : Limitation of Earth Fault Current

Discrimination not required can limit current to very low value. Sometimes down to 5A

Earth faults on the power system are not seen by the generator earth fault protection.

Method of Earthing : Limitation of Earth Fault Current

Limit To Generator Full Load Current

• Most popular.

• Used for ease of fault detection and discrimination.

• Residual connection of CTs can be used

• Can result in serious core damage.

Directly Connected Generators :

Earthed Generator : Earth fault relay must be time delayed forco-ordination with other earth fault protection on the power system.

Unearthed Generators : Other generators connected in parallelwill generally be unearthed.

Protection is restricted to faults on the generator, grading with power system earth fault protection is not required. A high impedance instantaneous relay can be used (Balanced Earth Fault protection).

51N50N

Percentage Winding Protected

250/1A IS

11.5kV; 75,000KVA

0.8x 250

1 x x.200 Ι

x.200 33

x.6600

operationFor

Y)S(SECONDAR

FS(PRIMARY)

For protection of 90% of winding; x = 1-0.9 = 0.1 Relay setting = 0.8 x 0.1 = 0.08A = 8% of 1A

Generators connected via step-up transformer (resistance earthed) :

Instantaneous protection (50N) :

System earth faults ARE not seen by generator earth fault protection instantaneous relay may be used.

Set to 10% of resistor rating (avoids operation due to transient surges passed through generator transformer interwinding capacitance).

Advantage : Fast

51N 50N

Time delayed protection (51N) :

Time delay prevents operation on transient surges.

A more sensitive current setting may be used.

Set to 5% of resistor rating.

Advantage : Sensitive

On large machines considered worthwhile to use both

instantaneous and time delayed.

Restricted Earth Fault Protection

Protects approx. 90 - 95% of generator winding.

Terminal CT Inputs

E/F CT Input

P342/3 Relay

2000/1 ?

500/1 ?

Connections for Biased REF

• Smaller rating machines may have only one (neutral) tail CT brought out for connection

0 1 2 3 4

Restrain

Operate

Biased REF Protection OperatingCharacteristic

• High sensitivity (5%)

• Unit Protection

• FASTEffective bias (x In) = Max. phase current + k . I

Differential current (x In)

= I + I + I + k . IA B C N

Neutral Displacement / ResidualOvervoltage - Earth Fault Protection

P340Relay

(1) Derived measurement from 5-limb or 3 x 1 phase VT

(2) Directly measured from a broken delta VT input

(3) Directly measured across an earthing resistor

• 100% Stator Earth Fault Protection :

• Standard relays only cover 95% of winding.

• Probability of fault occurring in end 5% is low.

• On large machines 100% stator earth fault protection may be required.

• Two methods :

− Low Frequency Injection− Third Harmonic Voltage Measurement

100% Stator Earth Fault Protection (27TN)

(1) Derived measurement from 5-limb or 3 x 1 phase VT(2) 3rd harmonic overvoltage(3) 3rd harmonic undervoltage

• 3rd harmonic undervoltage supervised by 3 phase undervoltage and W/VA/Var at generator terminals

P340Relay

100% Stator Earth Fault Protection

Distribution of 3rd harmonic voltage along the stator winding

• (a) normal operation

• (b) stator earth fault at star point

• (c) stator earth fault at the terminals

100% Stator Earth Fault - Low Frequency Injection

For Large Machines Only

InjectionTransformer

51 Alternative InjectionPoints

• Injection Frequency 12.5 - 20Hz

• Provides protection during run up & Standstill

• High cost due to injection equipment.

Overcurrent Protection

• For small generators this may be the only protection applied.

• With solid earthing it will provide some protection against earth faults.

• For a single generator, CTs must be connected to neutral end of stator winding.

• For parallel generators, CTs can be located on line side.

Differential Protection

• Provides high speed protection for all fault types

• May be : High impedance type : Biased (low impedance)

CT’s required in neutral end of winding

Differential Protection - Biased

OPERATE

BIASBIAS

Biased Differential Scheme

Differential Protection

Overall Differential Scheme

INTERPOSINGC.T.

Independent current settings per phase Single stage definite time delay

Interturn Protection (50DT)

Neutral Displacement / ResidualOvervoltage - Interturn Protection (59N)

GenRelay

(1) Interturn, derived measurement from 5-limb or 3 x 1 phase VT (2) Interturn, directly measured from a broken delta VT input (3) 95% stator earth fault protection across an earthing resistor

Prime Mover Failure

Isolated Generators :

Machine slows down and stops. Other protection initiates shut down.

Parallel Sets :

System supplies power - generator operates as a motor.Seriousness depends on type of drive.

Steam Turbine Sets :

Steam acts as a coolant.Loss of steam causes overheating.Turbulence in trapped steam causes distortion of turbine blades.Motoring power 0.5% to 6% rated.Condensing turbines, rate of heating slow. Loss of steam instantly recognised.

Prime Mover Failure

Diesel Driven Sets :

Prime mover failure due to mechanical fault.Serious mechanical damage if allowed to persist.Motoring power from 35% rated for stiff machine, to 5% rated for run in machine.

Gas Turbines :

Motoring power 100% rated for single shaft machine, 10% to 15% rated for double shaft.

Hydro Sets :

Mechanical precautions taken if water level drops.Low head types - erosion and cavitation of runner can occur.Additional protection may be required.

Prime Mover Failure

Reverse Power Protection :

Reverse power measuring relays used where protection required.

Single phase relay is sufficient as prime mover failure results in balanced conditions.

Sensitive settings required - metering class CTs required for accuracy.

Reverse Power

• Blinders at 0.5 degrees reduces operation area for low power settings where the power factor is low to improve reliability of reverse power element

Operational limits

Trip a rea

U nsta b le a reaU nsta b le a rea

as ta b le

n a tu ra la = 0 .1 6 o

-P= P0= 0 .5 o

Low Forward Power

• To reduce the risk of overspeed damage to steam turbine generators a low forward power element is used for interlocking the generator CB and excitation for non urgent trips (eg thermal protection, stator earth fault for high impedance earthing).

• Turbine steam valves are tripped immediatelay and when power output has reduced the generator CB and excitation are tripped.

Operational limits

Trip a rea

U nsta b le a reaExtend ed Trip a reaP

Trip a rea

a s ta b le = 0 .5 o

Loss of Excitation

Effects

Single Generator :

− Loses output volts and therefore load.

Parallel Generators :

−Operate as induction generator (> synch speed)−Flux provided by reactive stator current drawn from system-leading pf−Slip frequency current induced in rotor - abnormal

heating

Situation does not require immediate tripping,

however,

large machines have short thermal time constants - should be unloaded in a few seconds.

Loss of Excitation

Load Impedance

RImpedanceLocus

Offset – Prevents operation on pole

Diameter

Typically :Offset 50-75%X’dDiameter 50-100% XS

Time Delayed

Relay Characteristic

Impedance seen by relay follows locus shown below :

Pole Slipping

Sudden changes or shocks in an electrical power system may lead to power system oscillations - regular variations of I and V and angular system separation

In a recoverable situation these oscillations will die away - a power swing

In an unrecoverable situation the oscillations become so severe that synchronisation between the generator and the power system is lost - out of step/pole slipping

Causes− Transient system faults− Failure of the generator governor− Failure of the generators excitation

control− Reconnection of an islanded system

without synchronisation− Switching transients on a weak system

Pole slipping

Power Swing

Recoverable

Unrecoverable

Loss of Synchronism

Out-of-Step

(Power System)

Pole-Slipping

(Generator)

Theory of pole slipping

Where:

EG represents the generator terminal voltage;

ZG represents the generator reactance;

ZT is the reactance of step-up transform;

Zs represents the impedance of the power system connected to the generation unit

Es represents the system voltage.

Simplified Two Machine System:

Loss of synchronisation Characteristics

EG/ES<1

EG/ES>1EG/ES=1

Conventional Pole Slipping Protection

Reactance Line

Blinder

Zone 1

Zone 2

Pole Slipping Protection - 78

• Conventional lenticular (lens) characteristic− 2 Zones defined by reactance line− Zone 1 - pole slip in the generator− Zone 2 - pole slip in the power system− Separate counters per zone (1-20)

• Setting to detect pole slipping when :− Generating− Motoring− Both (Pumped storage generator)

Pole Slipping Protection - 78

• Pole slip when generating− Impedance position on RHS of lens characteristic− Impedance crosses lens on RHS− Impedance spends >T1 (15ms) in RHS of lens− Impedance spends >T2 (15ms) in LHS of lens− Impedance leaves lens on LHS − Zone 1 and 2 counter is incremented if in Z1− Zone 2 counter is incremented if in Z2− Trip when zone counter value exceeded

• Pole slipping when motoring is the opposite

State Transition Diagram

D E T E C T E D S T A R T

C O N F IR M

Zm = R 1 .R eset S tart_S igna ls;R eset F lag_Zone1;IF (Any T rip_S igna l) R eset C ounters; R eset T rip_S igna ls;

(Zm = R 4) & T im er2 > T2)If (C 2==0) S tart R eset_T im er;C 2++;Set Zone2_S tart;if(C 2>=C ount2) Set Zone2_Trip ;If (F lag_Zone1) C 1++; Set Zone1_S tart; if(C 1>=C ount1) Set Zone1_Trip ;R eset T im er2;

(Zm = R 3) & T im er1 > T1) F lag_Zone1=Zone1Pu();

R eset T im er1;S tart T im er2 ;

Zm = R 2Start T im er1

Zm = R 1 or R 2R eset F lag_Zone1;

R eset T im er2;

Zm = R 3 but T im er1<T1R eset T im er1

Zm = R 1 or R 3

Zm = R 2

Zm = R 3

Zm = R 4 or R 2 or R 3

(R eset_T im er T im e O ut)Actions are the sam e as

S ta te M ach ine EntryS tate M ach ine E ntry

R eset T rip_ S igna ls; R eset S tart_ S igna ls;

R eset F lag_Zone1;R eset A ll C ounters;

R eset A ll T im ers;

Zm = R 1 or R 4 R eset T im er1

Zm = R 4 bu t T im er2 < T2R eset F lag_Zone1;

R eset T im er2;

Zm = R 4IF (M ode_Both)F lag_M ode= !F lag_M ode;

*N o S igna l C ondition(V A<1V or I <0.02A )

N o S igna l C ondition*Actions are the sam e as

S ta te M ach ine Entry

V TS -FA S T-BLO C KActions are the sam e as

S ta te M ach ine Entry

RTDS Pole Slip Simulation

Local Load

T/line 140 km 11 kV BUS132/13.5 kV

Yd1Grid System Generator with

AVR and Governor control

132 kV BUS

Pole Slipping - 80% Load, Local 3 ph fault

Loss of excitation at 100% machine loading

Rotor ThermalProtection

• Unbalanced loading leads to negative sequence current

• Double frequency slip

• Rapid overheating of rotor

Unbalanced Loading

• Gives rise to negative phase sequence (NPS) currents - results in contra-rotating magnetic field

• Stator flux cuts rotor at twice synchronous speed

• Induces double frequency current in field system and rotor body

• Resulting eddy currents cause severe over heating− Use negative sequence overcurrent relay− Relay should have inverse time characteristic to

match generator I22t withstand

Unbalanced Loading

• Machines are assigned NPS current withstand values :−Continuous NPS rating, I2R (PU CMR)−Short time NPS rating, I22t (K)

• If possible level of system unbalance approaches machine continuous withstand, protection is required.

Overload Protection

high load current

heating of stator and rotor

insulation failure

Governor Setting

Should prevent serious overload automatically.Generator may lose speed if required load can not be met by other sources.

Stator Thermal Protection

Current operated−Over power protection−Overcurrent element −Thermal replica

RTD Thermal Probes−PT100 Platinum probes−Embedded in machine−Alarm and trip thresholds for each RTD

Current

Overload Protection (1)

• Thermal replica for stator overload protection− Current based on I1 and I2− Heating and cooling time constants− Non-volatile memory thermal state− Alarm output

Rotor Earth Fault Protection

Field circuit is an isolated DC system.

• Insulation failure at a single point :− No fault current, therefore no danger− Increase chance of second fault occurring

• Insulation failure at a second point :− Shorts out part of field winding− Heating (burning of conductor)− Flux distortion causing violent vibration of rotor

• Desirable to detect presence of first earth fault and give an alarm.

Exciter

Potentiometer Method

• Required sensitivity approximately 5% exciter voltage.

• No auxiliary supply required.

• “Blind spot” - require manually operated push button to vary tapping point.

AC Injection Method

• Brushless Machines

• No access to rotor circuit

• Require special slip rings for measurement

• If slip rings not present, must use telemetering techniques (expensive)

AC AuxiliarySupply

Brushless MachineA brushless generator has an excitation system consisting of:

− A main excitor with rotating armature and stationary field windings− A rotating rectifier assembly, carried on the main shaft line out− A controlled rectifier producing the d.c. field voltage for the main exciter field

from the a.c. source (often a small `pilot` exciter)

Hence:− No brushes are required in the field circuit− All control is carried out in the field circuit of the main exciter− Detection of rotor circuit earth fault is still necessary− Based on dedicated rotor-mounted system that has a telemetry link to provide

an alarm/data

Generator Back-Up Protection

10 x FL

with AVR

no AVR

Cycles

FullLoad

Typical use :− Very or extremely inverse for LV machines− Normal inverse for HV machines

Must consider generator voltage decrement characteristic for close-in faults.With reliable AVR system, “conventional” overcurrent relays may be used.Otherwise, voltage controlled / restrained relays are required.

Generator Back-Up Protection

Voltage Restrained

• Operating characteristic is continuously varied depending on measured volts.

• Alternatively, use impedance relay.

Voltage Controlled

• Relay switches between fault characteristic and load characteristic depending on measured volts.

O/L CHARAC

FAULT CHARAC1.0

GENERATOR DECREMENTCURVE

0.01 10

0AMPS10,0003000100

LARGESTOUTGOING FEEDER

5MVA115% XS

500/5200/5

Generator Back-Up Protection (2)

Terminal Volts

LoadFault

Voltage control

Terminal Volts

LoadFault

Voltage restraint

Voltage Dependent Overcurrent Protection (51V)

Impedance Relay

• 2 Zones of protection− Zone 1 - Set to operate at 70% rated load impedance.

Back-up protection for generator-transformer, busbar and outgoing feeders. Time delayed for co-ordination with external feeder phase fault protection.

− Zone 2 – Set to 50% transformer impedance. Back-up protection for generator phase faults. Faster time delay to co-ordinate with generator phase fault protection

LoadFault

Underimpedance

Under & Over Frequency Conditions

Over Frequency

• Results from generator over speed caused by sudden loss of load.

• In isolated generators may be due to failure of speed governing system.

• Over speed protection may be provided by mechanical means.

• Desirable to have over frequency relay with more sensitive settings.

Under & Over Frequency Conditions

Under Frequency

• Results from loss of synchronous speed due to excessive overload.

• In isolated generators may be due to failure of speed governing system.

• Under frequency condition gives rise to:−Overfluxing of stator core at nominal volts−Plant drives operating at lower speeds - can affect

generator output−Mechanical resonant condition in turbines

• Desirable to supply an under frequency relay.• Protection may be arranged to initiate load

shedding as a first step.

df/dt+t: Time Delayed ROCOF

• Df/dt can operate quicker than underfrequency for large changes in frequency

• Rolling window is better than fixed window as gives faster operation

• Averaging cycles is typically 5 to provide some stability for power system oscillations

• Stages can be used for load shedding or alarm/tripping of the generator

df/dt (81R)Loadshedding

Under & Over Voltage Conditions

Protection

• Under & over voltage protection usually provided as part of excitation system.

• For most applications an additional high set over voltage relay is sufficient.

• Time delayed under and over voltage protection may be provided.

Under & Over Voltage Conditions

Over Voltage

• Results from generator over speed caused by sudden loss of load.

• May be due to failure of the voltage regulator.

• An over voltage condition :

− Causes overfluxing at nominal frequency− Endangers integrity of insulation

Under Voltage

• No danger to generator. May cause stalling of motors.

• Prolonged under voltage indicates abnormal conditions.

Generator Abnormal Frequency Protection (81AB)

• 6 independent bands of abnormal frequency protection

• Accumulation of time up to 1000 hours in each band

• Band data provided by generator manufacturer

• Bands match resonance, blade stress frequencies …

• Dead band timer before accumulation starts allows time for resonance to established

• When generator is off-line bands can be blocked

Generator Abnormal Frequency Protection (81AB)

Band 1f nom

Band 4

Band 3

Band 2

Timer 1

Timer 2

Timer 3

Timer 4

Application Negative Sequence Overvoltage (47)

Generator/MotorCB

Negative Sequence Overvoltage

Swapping of 2 phases to motor (pump water)

Generator/Motor

Block CB Close

Busbar

Hydro machines can operate as motors/pumps by swapping 2 phases (phase rotation is reversed)

Generator differentialUnder & over voltageUnder & over frequencyReverse powerStator earth faultLoss of excitationVoltage dependent overcurrentNegative phase sequence

87G27 & 5981U & 81O32R51N4051V46

When the units are being used to generate power the protection could be as below:

When the units pump water the protection applied will change

2 31 4

Four groupsavailable

32R Reverse power

Use of Alternative Setting GroupsExample : Pumped Storage Unit

Phase Rotation

• Phase rotation for hydro generator/motor applications where 2 phases are swapped to make the machine operate as a pump (motor)

PhaseReversalSwitches

CT1 CT2

Case 1 : Phase Reversal Switches affecting all CTs and VTs

P343/4/5

PhaseReversalSwitches

CT1 CT2

Case 2 : Phase Reversal Switches affecting CT1 only

Phase Rotation

• Phase rotation settings can be changed for generator/motor operation using 2 setting groups

Setting Range Default SYSTEM CONFIG Phase Sequence Standard ABC /

Reverse ACB Standard ABC

VT Reversal No Swap / A-B Swapped / B-C Swapped / C-A Swapped

No Swap

CT1 Reversal No Swap / A-B Swapped / B-C Swapped / C-A Swapped

No Swap

CT2 Reversal (P343/4/5 only)

No Swap / A-B Swapped / B-C Swapped / C-A Swapped

No Swap

& Trip

Unintentional Energisation at Standstill

• Overcurrent element detects breaker flashover or starting current (as motor)

• Three phase undervoltage detection

• VTS function checks no VT anomalies

Check Synch (25)

• Check is used when closing generator CB to ensure synchronism with system voltage.

• Check synch relay usually checks 3 things:−Phase angle difference−Voltage−Frequency difference

Check Synchronising (25)

• Phase angle difference−Single phase comparison

• Can select either A-N, B-N, C-N, A-B, B-C, C-A is settings −Typical setting is 20º to reduce mechanical stresses on

generators.

• Voltage −Check synch relay inoperative if :-

• Generator/busbar voltage is below or above preset limit (independent settings for generator and busbar under/overvoltages)

• voltage difference exceeds preset limit−Typical settings for undervoltage: 80 - 85% Vn−Typical settings for difference voltage: 6 - 10% Vn

• Frequency difference−Usually measured by time to traverse phase angle limits or direct

slip frequency measurement (Fgen – Fbus)• Eg Timer setting of 2 secs over 20º :

• Slip frequency = 2 x (20 x ½) / 360 = 0.055Hz = 0.11% (50Hz)

• Timer usually set to 2 secs or 10 x C.B. closing time whichever is greater).

Check Synchronising (25)

• Check synch has 2 stages – Check Sync 1/2−Usually only 1 stage is required for generator

applications−Check Sync 2 has CB closing time compensation−Check Sync2 only permits closure for decreasing angles

of slip

• Check synch has vector compensation to account for phase shift across transformer with Main VT Vect Grp setting 0-11

• Check synch has ratio correction to correct ratio errors of VTs

• Voltage monitors for dead/live generator/busbar

• System Split output operates for phase angle > setting adjustable from 90 to 175 degrees

Check Synch (25)

Check synch stages 1 and 2

Typical Schemes

Protection Package for Diesel Generator

32 Reverse Power64R Rotor Earth Fault 64S Stator Earth Fault 51V Voltage Dependent

Overcurrent87G Generator Differential

Protection P343

Overall Protection of Generator Installation

Generator Feeder Protn.

Overcurrent Voltage

Restraint

Restricted

Buchholz Winding Temp.

Reverse Power

Field Failure

Generator Differential

Rotor E/F Prime Mover Protection

Negative Phase Sequence

Stator E/F

Overall Gen/Trans Diffl Protn.

Overall Protection of Generator Installation

Generator Feeder Protection

Low Steam Pressure, Loss of Vacuum

Loss of Lubricating OilLoss of Boiler Water

Governor FailureVibration, Rotor Distortion

O/C Circuit Breaker Fail

Busbar Protection

Restricted E/F

Buchholz Winding Temperature

V.T.sO/C

Transformer

Overfluxing

Restricted E/F

Standby E/F

BuchholzO/C + E/F

Unit Transformer Differential Protn.

Overall Generator

Transformer Differential

Protn.

Rotor E/F

Permissive (Low

Power) Interlock Pole

SlippingField Failure

Generator Differential

Negative Phase Sequence

Stator E/F Protection

Embedded Generation

PES system

Frequency

Voltage

Residual Voltage

59NIslanded load fed unearthed

O/C & E/F50/51N

Voltage Vector Shift

Co-generation/Embedded Machines

NPS Voltage

NPS O/C

Check Synch25

Embedded Generation

USED TO PROVIDE:

• Emergency Power Upon Loss Of Main Supply

• Operate In Parallel To Reduce Site Demand

• Excess Generation May Be Exported Or Sold

Engineering Recommendation G59

• ER G59 relates to the connection of generating plant to the distribution systems of licensed distribution network operators (DNOs)

• ER G83/1 covers connection of generating units rated < 16A / phase in parallel with LV distribution system

• ER G59 COVERS:−Safety Aspects−Legal Requirements−Operation−Protection

• The main function of the protection systems and settings is to prevent Generating Plant supporting an islanded section of the Distribution System when it would or could pose a hazard to the Distribution System or customers connected to it.

General Requirements Protective Equipment

• To disconnect the Generating Plant from the Distribution System in the event of loss of one or more phases of the DNOs supply.

• LoM is required to ensure requirements for earthing and out of synch closure are complied with and customers are not supplied with voltage and frequency outside statutory limits

LoM (Loss of Mains = Islanding) Protection Requirements

Loss of Mains Problem

• Loss of mains is where a generator is inadvertently isolated from the grid and continues to supply local load

• Loss of mains can be caused by:

−Protection tripping

−Accidentally due to network reconfiguration

33kV DISTRIBUTION

33/11kV

400V DG

SECTIONALISINGSWITCH

CIRCUITBREAKER BUS-BAR

Loss of Mains Problem

Islanding is unacceptable for a number of reasons:

−Safety risk - for example, through personnel working on the network under the assumption that no parts of the network are energised

−Stresses from out of synchronism re-closure

−Loss of system earth where the earth is on the star winding of a network transformer. This can cause problems for existing earth fault protection to detect earth faults if the system is unearthed.

−Utility is legally bound to maintaining quality of supply (frequency and voltage ) to local demand.

Existing LoM Methods – Performance Assessment

• Loss of mains performance can be assessed in terms of sensitivity and stability

• Sensitivity

− Smallest possible mismatch between local generation and the demand at the instant of islanding.

− Also referred to as non-detection zone

• Stability

− Stability for different fault types with varying duration and retained voltage at the point of measurement

• When designing LoM method objective is to have a small non detection zone and be stable for as many fault characteristics as possible

STABILITY

SENSITIVITY

Generator/demand Imbalance

Network faults

Existing Loss of Mains Methods

• Passive Methods

− Under/over frequency and voltage • Requires large change in load, time delayed

− Df/dt – rate of change of frequency• Sensitive, fast operating

− Voltage vector shift• Not as sensitive as df/dt, fast operating

− Direct inter-tripping

• Not load dependent, fast, expensive, signalling can be complex

• Active Methods

− Active frequency drift

− Reactive Error export

• There is an abundance of active methods proposed in the technical literature, however, their application in practice has been limited to date. The traditional protection philosophy of independence from other systems makes the introduction of these methods difficult.

Single phase line diagram showing generator parameters

Loss of Mains Methods – Voltage Vector Shift

Vector Diagram Representing Steady State Condition

Transient voltage vector change due to change in load current IL

I XI RV

I X”

Loss of Mains Methods - ROCOF

The rate of change of speed, or frequency, following a power disturbance can be approximated by:

df/dt =

where P = Change in power output between synchronised and islanded operation

f = Rated frequency

G = Machine rate MVA

H = Inertia constant

df/dt =

Two consecutive calculations must give a result above the setting threshold before a trip decision can be initiated

P341 df/dt calculation

3 cycle

n n - 3 cycle

df/dt+t: Time Delayed ROCOF

df/dt Setting

Trip Pick up cycles

Time delay

G59 Other Protection

• Neutral voltage

• Overcurrent

• Earth fault

• Phase unbalance

• Reverse power − Used when generator does not export power during normal operation

G59 Protection Settings

Protection Settings for Long-Term Parallel Operation

Notes: K1 = 1.0 (low impedance networks or 1.66-2 (high impedance networks)K2 = 1.0 (low impedance networks or 1.6 (high impedance networks)A fault level of < 10% system design max fault level is high impedance* Might need to be reduced if auto-reclose time <3s

Prot Function Small Power Station Medium Power Station

LV Connected HV Connected

Setting Time Setting Time Setting Time

U/V st 1 Vph-n -13% 2.5s* Vph-ph -13% 2.5s Vph-ph -20% 2.5s*

U/V st 2 Vph-n -20% 0.5s Vph-ph -20% 0.5s

O/V st 1 Vph-n +10% 1.0s Vph-ph +10% 1.0s Vph-ph +10% 1.0s

O/V st 2 Vph-n +15% 0.5s Vph-ph +13% 0.5s

U/F st 1 47.5Hz 20s 47.5Hz 20s 47.5Hz 20s

U/F st 2 47Hz 0.5s 47Hz 0.5s 47Hz 0.5s

O/F st 1 51.5Hz 90s 51.5Hz 90s 51.5Hz 90s

O/F st 2 52Hz 0.5s 52Hz 0.5s

LoM (Vector Shift) K1 x 6 degrees K1 x 6 degrees Intertripping expected

LoM (RoCoF) K1 x 0.125 Hz/s K2 x 0.125Hz/s Intertripping expected

G59 Protection for HV Generator connected to DNO HV System for Parallel Operation Only

G59 Protection for HV Generator connected to DNO HV System for Standby and Parallel Operation

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Generator protection

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